Upgrade to Webpack 4

[blairg]

When attempted, get this error: -

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.

• configuration.module has an unknown property 'loaders'. These properties are valid:
  object { exprContextCritical?, exprContextRecursive?, exprContextRegExp?, exprContextRequest?, noParse?, rules?, defaultRules?, unknownContextCritical?, unknownContextRecursive?, unknownContextRegExp?, unknownContextRequest?, unsafeCache?, wrappedContextCritical?, wrappedContextRecursive?, wrappedContextRegExp?, strictExportPresence?, strictThisContextOnImports? }
  -> Options affecting the normal modules (NormalModuleFactory).